1. Field of Invention
The present invention concerns an inprovement in the method of refining, particularly decarburization of molten steel. The invention also concerns an apparatus for carrying out this method of refining.
The term "molten steel" in this specification means, not only metal mixtures having composition of steel at the initial stage of melting, but also those which may have a composition of steel as the result of the refining, such as pig iron.
2. State of the Art
In the processes for refining by blowing oxygen gas into the molten steel in an vessel such as a conventor, AOD furnace and VOD furnace, to cause decarburization and other refing reactions, it is essential to accurately determine the carbon content in the molten steel and to lead the final carbon content to a desired level. Also, in the processes for refining using an inert gas such as argon gas in addition to oxygen gas, with the view to reduce manufacturing costs, it is quite preferable to choose the most suitable flow rates and mixing ratios of the gases on the basis of the carbon content in the molten steel at various stages of the decarburization. If the expected optimum operation could be carried out to shorten the necessary period of the refining, it would be possible to increase time of using the vessel which is at present limited by the life of refractories.
As a matter of fact, it is not easy to determine the optimum pattern of the operation. The only way now available to find out a better operation pattern relies upon experiences resulted from trial-and-error. For example, in the refining of a stainless steel by AOD process, blowing gasses is interrupted at the first, the second and the third stages of the decarburization period when the carbon content of the molten steel reaches 0.30%, 0.10 % and the target level, respectively, the flow rates of oxygen gas and the inert gas (therefore, also the mixed ratio thereof) is changed prior to proceeding the subsequent stage, and, after completion of the decarburization period, chromium-reducing period comes.
The operation pattern as described above should be determined in each case on the basis of the composition of the steel to be refined. However, no effective way of deciding the operation pattern has been found. Furthermore, it has been necessary to take samples of the molten steel for chemical analysis at every stages and to measure the temperature. It is of course preferable to decrease the necessity of sampling and temperature measurement, and save time and labor.
Under the circumstances, there has been demand for the way of determining the carbon content in the molten steel, which is decreasing due to the decarburization, with decreased number of chemical analysis, and of improving rate of getting right carbon content as desired.
As a method of assuming the carbon content in the molten steel during a steelmaking process with decarburization, there was disclosed the technology to measure contents of CO and CO.sub.2 in the exhaust gas from the refining vessel by chemical analysis to calculate the total amount of carbon deprived of the molten steel on the basis of the total volume of the exhaust gas, thereby to determine the amount of remaining carbon (Japanese Patent Disclosure No. 54-42323). Also, it was proposed to recognize the pattern of decarburization rate based on the chemical analysis of the exhaust gas to improve the rate of getting the right final carbon content (Japanee Patent Disclosure No. 54-53612).
It is a prerequisite for use calculation of the amount of the removed carbon based on the chemical analysis of the exhaust gas that not only the composition but also the flow rate of the exhaust gas is acculately measured. This is, however, not easy in general, and particularly difficult in applying to the refining in which an open type refining vessel is used.